U.S. Pat. No. 4,939,316 to Mahulikar et al discloses an adhesively sealed package for housing an electronic device such as a semiconductor integrated circuit. The base and cover are formed from aluminum or an aluminum alloy. Disposed between the base and cover is a leadframe. An adhesive such as epoxy bonds the leadframe to both. At least those surfaces exposed to the atmosphere are anodized. Preferably, the surfaces which contact the adhesive are also anodized.
The anodized surface is roughened. A roughened surface increases the mechanical locking of the adhesive to improve bond strength. The anodization layer further provides salt spray corrosion protection for those surfaces exposed to the atmosphere.
The anodization layer is colored light black or black. The black color maximizes infrared (IR) absorption. IR heating is used to solder the leadframe to a printed circuit board or other external circuitry. Colors other than black have high IR reflectivity. The leads do not heat to the soldering temperature. The black color also has cosmetic value. Anodized aluminum packages supplement molded plastic packages which are generally colored black.
The thickness of the anodization film determines the thermal and electrical performance of the package. If the film is too thick, conduction of heat from a mounted semiconductor chip will be inadequate. Any rise in the operating temperature of the integrated circuit device decreases the functional life. A thinner anodization layer results in better thermal performance. Balanced against thermal performance is electrical insulation and corrosion resistance. The anodization film must be sufficiently thick to ensure good electrical insulation both between the exterior surfaces of the package and adjoining circuitry and the interior of the package and the device. Aluminum alloys are corroded by salt spray. The anodization layer must be sufficiently thick to resist corrosion.
The surface roughness of the anodization layer must be sufficient to provide good mechanical adhesion for the epoxy. The roughness cannot detract from the cosmetic appearance of the part or expose aluminum base metal.
Additionally, the anodization film must be sufficiently strong to tolerate assembly operations and post assembly testing and handling. Chemical resistance is required if the package is to be immersed in a plating solution such as to tin plate the leads. The anodization layer must be sufficiently strong and adherent to resist thermal stresses caused by the heating of the electronic device during IR soldering and thermal cycling. The corrosion resistance must be sufficient to withstand salt spray testing.
Anodization is usually clear or colored with a slight golden tinge. There are several ways to produce a more intense color. An organic dye can be impregnated in the pores of the anodic coating. Mineral pigments, such as iron oxide, can be precipitated in the pores. Both methods have limitations. It is difficult to get a uniform color from part to part. The coloring tends to fade or bleed over time.
A preferred method of coloring is integral color anodization. Pigmentation is caused by a variety of factors including the occlusion of micro particles in the coating. The micro particles are a product of the anodic reaction of the electrolyte with the constituents of the aluminum alloy. Alloy composition and temper strongly affect the color produced.
Integral color anodization is disclosed in U.S. Pat. No. 3,669,855 to Smith and U.S. Pat. No. 3,714,000 to Dorsey, Jr. U.S. Pat. No. 3,669,855 discloses an aqueous electrolyte having 64.6 gm/l sulfosalicylic acid, 5.8 gm/l sulfuric acid and 1.7 gm/l aluminum. Color is controlled by developing a voltage/time relationship. The amperage and plating time are controlled in accordance with the voltage/time relationship.
U.S. Pat. No. 3,714,000 discloses an electrolyte including an aromatic acid, such a benzoic or carboxylic acid.
U.S. Pat. No. 3,616,311 to Barkman discloses integral color anodization from a bath containing an inorganic acid such as sulfuric acid, a voltage component such as sulfonic acid and a metal salt of an organic acid for color control. The voltage is regulated to maintain a constant current of about 48 amps pert square foot.
The above processes do not satisfy the stringent requirements of an anodized aluminum package component. A controlled increase in amperage up to a steady state imparts a greenish color to the aluminum alloys and which is not acceptable for infrared heating. Hard anodizing at high voltages and high current densities produces a black corrosion resistant film. The film thickness is difficult to control leading to poor thermal characteristics.